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| 1 | +//===----------------------------------------------------------------------===// |
| 2 | +// |
| 3 | +// This source file is part of the Swift.org open source project |
| 4 | +// |
| 5 | +// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors |
| 6 | +// Licensed under Apache License v2.0 with Runtime Library Exception |
| 7 | +// |
| 8 | +// See https://swift.org/LICENSE.txt for license information |
| 9 | +// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors |
| 10 | +// |
| 11 | +//===----------------------------------------------------------------------===// |
| 12 | + |
| 13 | +import Foundation |
| 14 | + |
| 15 | +/// Abstraction layer so we can store a heterogeneous collection of tasks in an |
| 16 | +/// array. |
| 17 | +private protocol AnyTask: Sendable { |
| 18 | + func waitForCompletion() async |
| 19 | + |
| 20 | + func cancel() |
| 21 | +} |
| 22 | + |
| 23 | +extension Task: AnyTask { |
| 24 | + func waitForCompletion() async { |
| 25 | + _ = try? await value |
| 26 | + } |
| 27 | +} |
| 28 | + |
| 29 | +/// A type that is able to track dependencies between tasks. |
| 30 | +public protocol DependencyTracker: Sendable, Hashable { |
| 31 | + /// Whether the task described by `self` needs to finish executing before `other` can start executing. |
| 32 | + func isDependency(of other: Self) -> Bool |
| 33 | +} |
| 34 | + |
| 35 | +/// A dependency tracker where each task depends on every other, i.e. a serial |
| 36 | +/// queue. |
| 37 | +public struct Serial: DependencyTracker { |
| 38 | + public func isDependency(of other: Serial) -> Bool { |
| 39 | + return true |
| 40 | + } |
| 41 | +} |
| 42 | + |
| 43 | +package struct PendingTask<TaskMetadata: Sendable & Hashable>: Sendable { |
| 44 | + /// The task that is pending. |
| 45 | + fileprivate let task: any AnyTask |
| 46 | + |
| 47 | + /// A unique value used to identify the task. This allows tasks to get |
| 48 | + /// removed from `pendingTasks` again after they finished executing. |
| 49 | + fileprivate let id: UUID |
| 50 | +} |
| 51 | + |
| 52 | +/// A list of pending tasks that can be sent across actor boundaries and is guarded by a lock. |
| 53 | +/// |
| 54 | +/// - Note: Unchecked sendable because the tasks are being protected by a lock. |
| 55 | +private final class PendingTasks<TaskMetadata: Sendable & Hashable>: Sendable { |
| 56 | + /// Lock guarding `pendingTasks`. |
| 57 | + private let lock = NSLock() |
| 58 | + |
| 59 | + /// Pending tasks that have not finished execution yet. |
| 60 | + /// |
| 61 | + /// - Important: This must only be accessed while `lock` has been acquired. |
| 62 | + private nonisolated(unsafe) var tasksByMetadata: [TaskMetadata: [PendingTask<TaskMetadata>]] = [:] |
| 63 | + |
| 64 | + init() { |
| 65 | + self.lock.name = "AsyncQueue" |
| 66 | + } |
| 67 | + |
| 68 | + /// Capture a lock and execute the closure, which may modify the pending tasks. |
| 69 | + func withLock<T>( |
| 70 | + _ body: (_ tasksByMetadata: inout [TaskMetadata: [PendingTask<TaskMetadata>]]) throws -> T |
| 71 | + ) rethrows -> T { |
| 72 | + try lock.withLock { |
| 73 | + try body(&tasksByMetadata) |
| 74 | + } |
| 75 | + } |
| 76 | +} |
| 77 | + |
| 78 | +/// A queue that allows the execution of asynchronous blocks of code. |
| 79 | +public final class AsyncQueue<TaskMetadata: DependencyTracker>: Sendable { |
| 80 | + private let pendingTasks: PendingTasks<TaskMetadata> = PendingTasks() |
| 81 | + |
| 82 | + public init() {} |
| 83 | + |
| 84 | + /// Schedule a new closure to be executed on the queue. |
| 85 | + /// |
| 86 | + /// If this is a serial queue, all previously added tasks are guaranteed to |
| 87 | + /// finished executing before this closure gets executed. |
| 88 | + /// |
| 89 | + /// If this is a barrier, all previously scheduled tasks are guaranteed to |
| 90 | + /// finish execution before the barrier is executed and all tasks that are |
| 91 | + /// added later will wait until the barrier finishes execution. |
| 92 | + @discardableResult |
| 93 | + public func async<Success: Sendable>( |
| 94 | + priority: TaskPriority? = nil, |
| 95 | + metadata: TaskMetadata, |
| 96 | + @_inheritActorContext operation: @escaping @Sendable () async -> Success |
| 97 | + ) -> Task<Success, Never> { |
| 98 | + let throwingTask = asyncThrowing(priority: priority, metadata: metadata, operation: operation) |
| 99 | + return Task(priority: priority) { |
| 100 | + do { |
| 101 | + return try await throwingTask.valuePropagatingCancellation |
| 102 | + } catch { |
| 103 | + // We know this can never happen because `operation` does not throw. |
| 104 | + preconditionFailure("Executing a task threw an error even though the operation did not throw") |
| 105 | + } |
| 106 | + } |
| 107 | + } |
| 108 | + |
| 109 | + /// Same as ``AsyncQueue/async(priority:barrier:operation:)`` but allows the |
| 110 | + /// operation to throw. |
| 111 | + /// |
| 112 | + /// - Important: The caller is responsible for handling any errors thrown from |
| 113 | + /// the operation by awaiting the result of the returned task. |
| 114 | + public func asyncThrowing<Success: Sendable>( |
| 115 | + priority: TaskPriority? = nil, |
| 116 | + metadata: TaskMetadata, |
| 117 | + @_inheritActorContext operation: @escaping @Sendable () async throws -> Success |
| 118 | + ) -> Task<Success, any Error> { |
| 119 | + let id = UUID() |
| 120 | + |
| 121 | + return pendingTasks.withLock { tasksByMetadata in |
| 122 | + // Build the list of tasks that need to finished execution before this one |
| 123 | + // can be executed |
| 124 | + var dependencies: [PendingTask<TaskMetadata>] = [] |
| 125 | + for (pendingMetadata, pendingTasks) in tasksByMetadata { |
| 126 | + guard pendingMetadata.isDependency(of: metadata) else { |
| 127 | + // No dependency |
| 128 | + continue |
| 129 | + } |
| 130 | + if metadata.isDependency(of: metadata), let lastPendingTask = pendingTasks.last { |
| 131 | + // This kind of task depends on all other tasks of the same kind finishing. It is sufficient to just wait on |
| 132 | + // the last task with this metadata, it will have all the other tasks with the same metadata as transitive |
| 133 | + // dependencies. |
| 134 | + dependencies.append(lastPendingTask) |
| 135 | + } else { |
| 136 | + // We depend on tasks with this metadata, but they don't have any dependencies between them, eg. |
| 137 | + // `documentUpdate` depends on all `documentRequest` but `documentRequest` don't have dependencies between |
| 138 | + // them. We need to depend on all of them unless we knew that we depended on some other task that already |
| 139 | + // depends on all of these. But determining that would also require knowledge about the entire dependency |
| 140 | + // graph, which is likely as expensive as depending on all of these tasks. |
| 141 | + dependencies += pendingTasks |
| 142 | + } |
| 143 | + } |
| 144 | + |
| 145 | + // Schedule the task. |
| 146 | + let task = Task(priority: priority) { [pendingTasks] in |
| 147 | + // IMPORTANT: The only throwing call in here must be the call to |
| 148 | + // operation. Otherwise the assumption that the task will never throw |
| 149 | + // if `operation` does not throw, which we are making in `async` does |
| 150 | + // not hold anymore. |
| 151 | + for dependency in dependencies { |
| 152 | + await dependency.task.waitForCompletion() |
| 153 | + } |
| 154 | + |
| 155 | + let result = try await operation() |
| 156 | + |
| 157 | + pendingTasks.withLock { tasksByMetadata in |
| 158 | + tasksByMetadata[metadata, default: []].removeAll(where: { $0.id == id }) |
| 159 | + if tasksByMetadata[metadata]?.isEmpty ?? false { |
| 160 | + tasksByMetadata[metadata] = nil |
| 161 | + } |
| 162 | + } |
| 163 | + |
| 164 | + return result |
| 165 | + } |
| 166 | + |
| 167 | + tasksByMetadata[metadata, default: []].append(PendingTask(task: task, id: id)) |
| 168 | + |
| 169 | + return task |
| 170 | + } |
| 171 | + } |
| 172 | +} |
| 173 | + |
| 174 | +/// Convenience overloads for serial queues. |
| 175 | +extension AsyncQueue where TaskMetadata == Serial { |
| 176 | + /// Same as ``async(priority:operation:)`` but specialized for serial queues |
| 177 | + /// that don't specify any metadata. |
| 178 | + @discardableResult |
| 179 | + public func async<Success: Sendable>( |
| 180 | + priority: TaskPriority? = nil, |
| 181 | + @_inheritActorContext operation: @escaping @Sendable () async -> Success |
| 182 | + ) -> Task<Success, Never> { |
| 183 | + return self.async(priority: priority, metadata: Serial(), operation: operation) |
| 184 | + } |
| 185 | + |
| 186 | + /// Same as ``asyncThrowing(priority:metadata:operation:)`` but specialized |
| 187 | + /// for serial queues that don't specify any metadata. |
| 188 | + public func asyncThrowing<Success: Sendable>( |
| 189 | + priority: TaskPriority? = nil, |
| 190 | + @_inheritActorContext operation: @escaping @Sendable () async throws -> Success |
| 191 | + ) -> Task<Success, any Error> { |
| 192 | + return self.asyncThrowing(priority: priority, metadata: Serial(), operation: operation) |
| 193 | + } |
| 194 | +} |
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